Question about a Collaboration feature

Hey All! So a favorite app of mine (GoodNotes) just added a collaboration feature. This is similar to iWork and other apps where you can invite users to work on the same project as you. My question is this: Does GoodNotes need to pay for server space on AWS/Azure to make this collaboration happen or is all of this using iCloud as the pipeline with the files being stored on each user's device and auto-updating as they work the projects?? I ask because I'm working on an app and am unsure how this type of feature works technically. Thanks in advance for your wisdom.

submitted by /u/Infoboy2u
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